A piston ring is an open-ended ring that fits into an annular groove in an outer circumference of a piston of a reciprocating engine, such as an internal combustion engine. When disposed within a cylinder of the engine, the piston ring is compressed around the piston and the intrinsic spring force of the ring ensures sufficient contact between an outer circumferential surface of the ring and an inner wall of the cylinder. During engine operation, the piston moves up and down within the cylinder and the pressure exerted on the cylinder wall by the piston ring provides a seal around the piston between the combustion chamber and the crankcase, which helps prevent escape of combustion gases from the combustion chamber and helps control oil consumption. Piston rings also may help transfer heat away from the piston to the cylinder wall and may be configured to regulate the amount of oil on the cylinder wall by distributing it on and wiping it off the cylinder wall.
Metallic and/or ceramic coatings may be applied to contact surfaces of various sliding components to improve the tribological properties thereof. For example, chromium platings are commonly applied to the outer circumferential surfaces of piston rings for increased wear resistance. Graphite, phosphate, or molybdenum coatings may be used to reduce the sliding friction between an outer contact surface of a piston ring and an interior surface of a cylinder wall.